This invention relates to the fabrication of semiconductor devices, and more particularly to the thermal growth of a stable oxide film or layer on the surface of a semiconductive silicon substrate. In accordance with a specific aspect, the invention relates to the fabrication of an insulated gate field-effect transistor in which the insulation layer comprises a film of silicon dioxide covered with a film of silicon nitride.
In the fabrication of microelectronic silicon devices, the formation of a silicon oxide film at various stages of wafer processing is a very common expedient. Primarily the oxide film is useful to exert a passivating influence on the silicon surface, both during manufacture and thereafter, to stabilize the electronic characteristics of a completed device. The oxide film is also used as a mask for the selective diffusion of conductivity type-determining impurities into the silicon substrate surface.
An early patent directed to various techniques for the controlled oxidation of silicon surfaces is U.S. Pat. No. 2,802,760 issued to Lincoln Derick and Carl Frosch. In one embodiment of the Derick and Frosch disclosure, a monocrystalline silicon wafer is subjected to oxidation at 1200.degree.C in an atmosphere consisting essentially of hydrogen and water vapor. Other embodiments include the use of an atmosphere consisting essentially of nitrogen and water vapor, or an atmosphere consisting essentially of oxygen and nitrogen or helium. The use of hydrogen as a carrier for oxygen was presumably unthinkable, due to its well-known character as a reducing agent, or perhaps due more simply to a fear of explosions. The oxide layer grown in accordance with such processes are subject to substantial deterioration under prolonged heat cycling. Moreover, such layers have an objectionable susceptibility to gradual change in electrical character under the influence of electrical bias. A migration of sodium ions or other contaminants to the siliconsilicon dioxide interface is presumably responsible for such change.
These adverse effects of sodium ion contamination are particularly severe in a silicon dioxide passivated insulated gate field-effect transistor. For example, if an N-channel MOSFET is operated with a continuous positive gate voltage--in the "on" state--the sodium ions begin to migrate. Since the ions are positive, a positive gate voltage drives them to the silicon dioxide-silicon interface. If the applied voltage is now reduced below the gate threshold, the induced N-channel should disappear. However, the layer of positive sodium ions adjacent the interface attracts a number of electrons to the silicon side of the interface. As a result the threshold voltage of such a device is substantially reduced. Thus, sodium ion contamination in the oxide layer renders the actual gate threshold unpredictable and dependent upon the previous history of the device.